1. Field of the Invention
This invention relates to a process for making field accessed bubble domain devices and to a device which permits manipulation, e.g. transfer or replication, of magnetic bubble domains between different propagation paths in typical magnetic bubble domain chip organization.
2. Description of Prior Art
In fabricating bubble domain devices, many processes are known. Frequently used processes are the so-called "two-level" or "one-level" processes. It is desirable to use as few "levels" of processing as possible in order to reduce fabrication complexity and to improve yields. Thus, the one-level process is frequently used in order to produce devices in which the permalloy overlay circuit serves as both the source of the bubble drive field and as a conductor of control currents for functions such as generation, annihilation, switching and the like. However, such devices suffer from certain drawbacks. For example, permalloy has a high resistivity (relative to the conductor material) wherein narrow control conductors can cause overheating and device burnout. Also, return current leads frequently interfere with bubble domain propagation in other components. Moreover, non-magnetic return current leads are sometimes necessary (or desirable) for proper operation of devices or systems. Consequently, a one-level process which overcomes these problems, and devices fabricated by such a process are desirable.
Devices or systems that utilize magnetic bubbles for storage of data in a binary form typically use soft magnetic overlay structures for propagation and storage of bubbles. These structures are usually arranged in arrays of T-I, X-T or chevron patterns. Transferring and replication (by stretching and separation of bubbles) between different tracks is accomplished by selectively interconnecting these tracks. Current carrying conductors are used to effect the transfer or the replicate functions in non-passive switches. Examples of these gates or switches are the so-called dollar sign transfer gate and the chevron-chevron transfer-replicate switch.
The dollar sign transfer gate disclosed in U.S. Pat. No. 3,714,639 to Kish et al., selectively diverts bubbles from T-I storage tracks and T-I access tracks when a current pulse is applied to the conductor. The dollar sign transfer gate is fabricated using a two step process in which the conductor level must be processed before the permalloy layer. These layers must be aligned to each other with a high degree of accuracy.
The chevron-chevron transfer replicate switch as described by T. J. Nelson, AIP Conference Proceedings, No. 18, Part 1, pp. 95-99 (1974) is used to transfer bubbles from one chevron path to another, such as between major and minor loops. This switch can be fabricated using a single level definition. One fine-definition process is needed to define both the permalloy and the conductor patterns. Chevron tracks, however, are not as compact as the T-I or T-X tracks such that fewer bubbles are stored for the same device surface area. Consequently, the device described by Nelson requires extensive chip geometry utilization and relatively high time requirements for data (bubble domain) throughput.